1. Field of the Invention
The present invention relates to improved longitudinal (horizontal) magnetic recording media and recording devices incorporating the media. In particular, the present invention relates to longitudinal recording media having a magnetic recording layer and a magnetically soft underlayer (SUL). The longitudinal recording media can advantageously be used with a perpendicular type of write head, such as a shielded pole write head, to achieve a high areal density.
2. Description of Related Art
Thin film magnetic recording media are composed of multiple layers, including one or more magnetic recording layers, disposed on a substrate. Typically, the magnetic recording layer includes small magnetic grains that have an easy magnetization axis that is magnetically oriented longitudinally (i.e., in plane) with respect to the layer.
The areal density of longitudinal magnetic recording media has been increasing at a compounded growth rate of about 60% per year and areal densities as high as 100 Gbit/in2 have been demonstrated. Scaling longitudinal recording media to higher areal densities requires smaller magnetic grains. However, as the grain size is reduced, thermal fluctuations can cause the magnetic domains to “flip”, causing a loss of magnetization over a period of time. Higher coercivity media and an increased track density (tracks per inch, or TPI) can mitigate this problem. However, the large write head gaps that are needed for good overwrite of high coercivity media lead to excessive fringing, negatively affecting the data written on adjacent tracks. The large write head gaps also reduce the write field gradient and thus lead to increased transition jitter (a component of noise) and the requirement of large track width parameters, decreasing the obtainable areal density.
Longitudinal magnetic recording media with soft magnetic layers have been described in the art. U.S. Pat. No. 5,041,922 by Wood et al. discloses a longitudinal magnetic recording medium that includes a hard magnetic layer and a magnetically saturable, high permeability, low coercivity (soft) magnetic layer. During signal reproduction (reading), the bias flux from the read transducer forms a saturation zone in the saturable layer that directs signal flux between the medium and the transducer.
U.S. Pat. Nos. 5,176,965 and 5,431,969 both by Mallary disclose a magnetic medium for longitudinal recording. The medium includes a thin magnetic image underlayer, a magnetic recording layer and a non-magnetic buffer layer disposed between the image underlayer and the magnetic recording layer. Charges located in the magnetic layer induce virtual magnetic images of opposite charge in the underlying magnetic image underlayer. The virtual image charges reduce the fringing effect in adjacent tracks during read operations. It is disclosed that the magnetic image layer can be magnetically soft, semi-soft or semi-hard.
In addition to longitudinal recording, perpendicular (vertical) magnetic recording media have been proposed as a way to increase areal densities. Perpendicular magnetic recording media include a magnetic recording layer having an easy magnetization axis that is perpendicular to the layer. A perpendicular write head, such as a monopole write head or a shielded pole write head, is utilized to magnetize the grains in the perpendicular recording layer. Examples of perpendicular recording media and perpendicular write heads are disclosed in U.S. Pat. No. 4,656,546 by Mallary and U.S. Pat. No. 4,748,525 by Perlov.
Among the desirable properties for the magnetic recording layer is a high coercivity. Coercivity is a measure of the magnetization field that must be applied to reduce the remnant magnetization to zero, i.e., to reverse the direction of magnetization. A high coercivity assures that the magnetic layer will have a high resistance to demagnetization by stray magnetic fields and will have good thermal stability. However, high coercivity media also require a high field strength to reverse the direction of magnetization, making the media difficult to write.
Therefore, there is a need for a longitudinal recording medium having an increased coercivity and an associated read/write device effective to form a strong field gradient across the magnetic material. There is also a need for a longitudinal recording medium having a high areal density that is capable of producing an acceptably high signal-to-noise ratio (SNR).